Synthesis of a Li- and Ta-Modified (K,Na)NbO 3 Solid Solution by Mechanochemical Activation Tadej Rojac, w Andreja BenWan, Hana UrgiW, Barbara MaliW,* and Marija Kosec* Jomef Stefan Institute, SI-1000, Ljubljana, Slovenia The synthesis of (K 0.485 Na 0.485 Li 0.03 )(Nb 0.8 Ta 0.2 )O 3 from a mechanochemically activated powder is presented for the first time. Mechanochemical activation, followed by a single calci- nation step was found to be a powerful technique for synthesiz- ing a homogeneous solid solution with a low amount of contaminant, coming from the milling equipment, and good electrical properties without the need for a multiple calcination procedure involving intermediate wet-milling steps, which are typically required in the conventional solid-state synthesis route. I. Introduction S INCE the discovery of the large piezoelectric coefficients in the (K,Na,Li)(Nb,Ta)O 3 ceramics 1 , extensive research has been carried out on the synthesis and characterization of lead-free (K,Na)NbO 3 -modified (KNN) solid solutions. However, there are still some common problems related to the synthesis of such compositions, i.e., the sensitivity of the preparation technique to the humidity of the environment due to the hygroscopic nature of the starting alkaline carbonates, 2 the difficulty in obtaining chemically homogeneous ceramics, 3,4 the volatilization of alkali oxides, and the strong tendency toward abnormal grain growth. 5 The difficulties encountered in the processing result in the poor reproducibility of the final electrical properties, being one of the major drawbacks with respect to industrial produc- tion. For this reason, further research must be carried out in order to develop a reliable processing route. Recently, special attention has been given to the composi- tional homogeneity as one of the key issues in the synthesis of KNN-based materials. 4 A homogeneous distribution, particu- larly of the B-site cations (Nb, Ta), is rather difficult to achieve using a conventional solid-state synthesis route and cannot be eliminated with prolonged high-temperature annealing and/or intensive milling. Therefore, a different approach was used, where Nb 2 O 5 and Ta 2 O 5 were prereacted to form a solid solu- tion. 4 Alternatively, (K 0.44 Na 0.52 Li 0.04 )(Nb 0.84 Ta 0.1 Sb 0.06 )O 3 was prepared by first synthesizing the binary compounds (KNbO 3 , KTaO 3 , NaNbO 3 , NaSbO 3 , and LiSbO 3 ), and then mixing them together to form the final composition. 2 However, these meth- ods require several processing steps, from the preparation of the precursors, and their mixing, to multi-calcinations with inter- mediate wet milling. Mechanochemical activation (increase in the reactivity of powders by high-energy milling) has recently received considerable interest for the processing of complex per- ovskite materials. 6 Its potential lies in the ability to prepare homogeneous nanopowders with enhanced reactivity by a sim- ple high-energy milling procedure, making it attractive especially for the preparation of complex compositions. The aim of the present work was to synthesize a (K 0.485 Na 0.485 Li 0.03 )(Nb 0.8 Ta 0.2 )O 3 solid solution from a mechanochemically activated powder. The compositional homogeneity, the structure, and the electrical properties of the obtained ceramics are presented. II. Experimental Procedure In order to synthesize a (K 0.485 Na 0.485 Li 0.03 )(Nb 0.8 Ta 0.2 )O 3 solid solution, subsequently referred to as KNLNT, K 2 CO 3 (991%, Aldrich, Sigma-Aldrich Chemie GmbH, Steinheim, Germany), Na 2 CO 3 (99.95%–100.05%, Aldrich), Li 2 C 2 O 4 (991%, Alfa, Alfa Aesar GmbH, Karlsruhe, Germany), Nb 2 O 5 (99.9%, Ald- rich), and Ta 2 O 5 (99%, Alfa) powders were used as the starting compounds. The K 2 CO 3 and Na 2 CO 3 powders were dried be- fore use at 2001C. After homogenization in a Fritsch Pulverisette 4 Vario-Mill (Fritsch GmbH, Idar-Oberstein, Germany) using acetone (50 g, 200 min À1 for 4 h) and drying, high-energy milling was per- formed for 10 h with the disk and the vial rotational frequencies set to 350 and 700 min À1 , respectively. A 250-mL tungsten car- bide vial, filled with 16 tungsten carbide milling balls with diameters of 15 mm, was used. After the high-energy milling, the powder was pressed into pellets, calcined at 8001C for 4 h, and sintered in air at 10801C for 2 h at a heating rate of 51C/min. The densities of the sintered pellets were determined with the Archimedes’ method. For the theoretical density (TD), a value of 5.0 g/cm 3 was used. 7 X-ray diffraction (XRD) patterns were recorded using a Pan- alitical X’Pert PRO diffractometer with CuKa 1 radiation (PAN- alytical B.V., Almelo, the Netherlands). The data were collected in the 2y range from 201 to 701 with a step of 0.0171/100 s and a fully opened X’Celerator detector. Thermogravimetric analysis (TG) was performed in flowing air at a heating rate of 101C/min using a NETZSCH STA 409 analyzer (Erich NETZSCH GmbH & Co. Holding KG, Selb, Germany). A simultaneous evolved-gas analysis (EGA) using a Balzers Thermostar GSD 300 T mass spectrometer (Balzers In- struments, Balzers, Liechtenstein) was also performed. The microstructures of the sintered samples were analyzed using a JEOL 5800 scanning electron microscope (SEM) (JEOL Ltd., Tokyo, Japan) equipped with a LINK ISIS 300 energy-dispersive X-ray spectrometer (EDXS). EDXS analyses were performed us- ing an acceleration voltage of 20 kV, a spectrum acquisition time of 100 s, a 351 take-off angle, and a 01 tilt of the specimen. Quantitative elemental analyses on K, Na, Li, Nb, and Ta on the sintered samples were performed using an inductively cou- pled plasma (ICP) atomic emission spectrometer Thermo Jarrel Ash Atomscan 25 (Thermo Jarrel Ash Corporation, Waltham, MA), while the amount of contaminant coming from the milling equipment (tungsten) was determined using an ICP mass spec- trometer 7500 ce (Agilent Technologies, Santa Clara, CA). The results were averaged over two parallel measurements. P. Bomlai—contributing editor The work was carried out as part of the EU 6FP IMMEDIATE, Network of Excellence MIND, Research Program ‘‘Electronic Ceramics, Nano, 2D and 3D Structures’’ P2-0105 and Postdoctoral project ‘‘Mechanochemical Synthesis of Complex Ceramic Oxides’’ Z2- 1195 (Slovenian Research Agency). *Member, The American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: tadej.rojac@ijs.si Manuscript No. 24547. Received April 17, 2008; approved August 11, 2008. J ournal J. Am. Ceram. Soc., 91 [11] 3789–3791 (2008) DOI: 10.1111/j.1551-2916.2008.02714.x r 2008 The American Ceramic Society 3789